RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, VIBRATIONAL ASSIGNMENT, BARRIERS TO INTERNAL-ROTATION AND AB-INITIO CALCULATIONS OF BUT-2-ENOYL CHLORIDE

The Raman (3500-1 0 cm-1) and infrared (3200-50 cm-1) spectra were rec orded for the fluid and solid phases of but-2-enoyl chloride (crotonyl chloride), trans-CH3CH-CHCClO, where the methyl group is trans to the CClO group, and a complete vibrational assignment is proposed. These data were interpreted on the basis that the s-trans (anti) form (two d ouble bonds oriented trans to one another) is the most stable form in the fluid phases and the only conformer remaining in the solid state. The asymmetric torsional fundamental of the more stable s-trans and th e higher energy s-cis (syn) form were observed at 97.5 and 86.9 cm-1, respectively. From these data the asymmetric potential function govern ing the internal rotation about the C-C bond was determined. The poten tial coefficients are V1 = - 111 +/- 2, V2 = 1860 +/- 48, V3 = 6 +/- 2 , V4 = - 43 +/- 24 and V6 = - 22 +/- 6. The s-trans to s-cis and s-cis to s-trans barriers were determined to be 1890 and 1785 cm-1, respect ively, with an enthalpy difference between the conformers of 105 +/- 5 2 cm - 1 1300 +/- 149 cal mol-1 (1 cal = 4.184 J) 1. Similarly, the ba rrier governing internal rotation of the CH3 group for the s-trans con former was also determined to be 912 +/- 30 (2.61 +/- 0.09 kcal mol-1) from the torsional fundamental observed in the far-infrared spectrum of the gas. All these data were compared with the corresponding quanti ties obtained from ab initio Hartree-Fock gradient calculations employ ing the RHF/3-21G, RHF/6-31G* and/or MP2/6-31G* basis sets. These res ults were compared with the corresponding quantities for some similar molecules.